Big Col ,the gearbox Jock refers to is on a vintage car, not a PVT, and he correctly describes it as a transaxle.
Jock, you are correct, there are two drain plugs - one for the axle, the other the gearbox. There is one filler which is on the rear axle diff casing. You have drained both units. The method is to fill the back axle to overflow point. You then have to take the car out (or raise it and run it, on axle stands or ramp, so that the axle is in use and back wheels are turning). The crown wheel works a bit like a water-wheel in scooping oil and depositing it in the gearbox top, it goes through a bush filling up the gearbox which has holes for overflowing and return to the axle when filled. It will take about 2-3 more pints.
I know it seems odd to be running the car with no oil in the gearbox but this is the method described by AC in the 1920's. If the car is on stands you can monitor the oil going into the box and top up as the level goes down. If taking the car out, take an oil pourer with you and I would suggest stop, firstly after say 3 or 4 miles and top up (so, some oil now in there), next stop say another 6-7 miles. This method normally requires c.3 stops to ensure the axle reaches overflow and the gearbox is full. The oil has to also travel down the half-shafts to the hubs and also down the torque tube (axle casing to universal joint, meeting the propshaft) - so a final top up could be required. Usually, the total oil is about 6+ pints for the transaxle unit.
